This invention relates to automatic control using proximity sensors, e.g. for controlling doors and the like. Its principal application is in the control of lift doors, though it may be used in various analogous applications, e.g. machine safety guards, vehicle reversing aids and the like.
For many years lift doors have been controlled by electromechanical apparatus. There has been a very substantial amount of effort directed to door control while nevertheless avoiding inconvenience to users of the lift. Many such systems rely on proximity sensors to detect the presence of a person or object which obstructs the door or would obstruct it. The problem with many such systems is that they work electronically and find difficulty in distinguishing between people and, for example, metal parts of the lift shaft construction.
PCT Patent Application No: PCT/GB 82/00022, Publication No: WO 82/02536, discloses proximity detector circuitry suitable for use in connection with lift doors. The systems described therein (and described in several of the Patent Specifications cited in the search report forming part of the publication) do represent workable systems. However, they suffer from a number of disadvantages, in particular, the necessity of fairly careful installation and balancing procedures, which are not always easy to effect on site, and additionally they tend to require in any practical application association with one or more microswitches which operate as the door moves. Mechanical components such as microswitches are costly to install and adjust.
We have now found that by using a somewhat different alternative approach, and using circuitry which can easily be constructed compactly and robustly, it is possible to produce improved lift door control circuits which will both detect the presence of a person or object obstructing the door or about to do so and act to interrupt or prevent a door closing operation and, further, which can detect the presence of a person or object adjacent the door and initiate a door opening operation.
One problem with many standard systems concerns the design of sensor antennae which are designed to extend along the edge of a door or edges of a pair of doors, that edge or edges being the leading edge(s) when the doors are closing. There are two separate problems which arise: First it is desirable that the antenna "looks" asymmetrically relative to the door. It is important to be able to detect people coming up to a lift door from outside in order to be able to abort a closing operation and reopen the doors, but it is equally important not to prevent closing because of the close proximity of people already safely inside the lift. In the past, asymmetric deep guard channels have been used partially to surround the antenna and give an asymmetric field of sensitivity. These take up space and are tricky to install and adjust.
A second problem is that the sensitivity of the signal picked up by the antenna is effectively heavily diluted by the capacitance between antenna and guard. This capacitance which can be quite high e.g. of the order of 100 picofarad, needs to be compared with the alteration in capacitance between antenna and ground when a body e.g. a person approaches the antenna. The change may be several orders of magnitude less, thus giving very small changes of potential on the antenna itself. The input signal to whatever subsequent circuitry is used is effectively severely attenuated.